Helicopters provide a flexible means of transporting supplies, passengers, and cargo over moderate distances at relatively high speeds. Unfortunately, the rotor blades of helicopters can produce a considerable amount of noise. In large part, the noise is “thickness noise” that is created by the blades as they move through the atmosphere. Thickness noise is produced by any body that moves through a fluid and that (of course) has some finite thickness. At its loudest, the thickness noise of a helicopter rotor blade noise originates near the tip of the advancing blade and is familiar to anyone who has heard the slapping sound of a nearby (or even distant) helicopter.
In large part because of thickness noise, the operation of helicopters is frequently restricted, or curtailed, over areas where people may be present, particularly over residential areas. The noise associated with helicopter landings and takeoffs also restricts the locations where helipads may be positioned. Thus, thickness noise constrains an otherwise versatile form of transportation. Thickness noise also plays the dominant role in the interior noise of tiltrotors when operated in airplane mode where the rotor serves as a propeller.